feat: implement security enhancements (task 17)

- Add rate limiting for login, registration, and invitations
- Implement CSRF protection with SameSite cookies and security headers
- Configure secure session cookies with httpOnly and secure flags
- Enhance input sanitization for XSS and injection prevention
- Add comprehensive security tests (41 tests passing)
- Document security enhancements in SECURITY_ENHANCEMENTS.md

Requirements: 9.3, 9.4, 9.5

lib/rate-limit.ts

@@ -0,0 +1,189 @@
+/**
+ * Rate limiting utilities for security
+ * Implements in-memory rate limiting for login attempts and other sensitive operations
+ * 
+ * Requirements: 9.3 - Rate limiting on authentication requests
+ */
+
+interface RateLimitEntry {
+  count: number
+  resetAt: number
+}
+
+// In-memory store for rate limiting
+// In production, consider using Redis or similar for distributed systems
+const rateLimitStore = new Map<string, RateLimitEntry>()
+
+// Cleanup interval to prevent memory leaks
+const CLEANUP_INTERVAL = 60 * 60 * 1000 // 1 hour
+setInterval(() => {
+  const now = Date.now()
+  const entries = Array.from(rateLimitStore.entries())
+  for (const [key, entry] of entries) {
+    if (entry.resetAt < now) {
+      rateLimitStore.delete(key)
+    }
+  }
+}, CLEANUP_INTERVAL)
+
+export interface RateLimitConfig {
+  /**
+   * Maximum number of requests allowed within the window
+   */
+  maxRequests: number
+  
+  /**
+   * Time window in milliseconds
+   */
+  windowMs: number
+  
+  /**
+   * Optional identifier (e.g., IP address, user ID)
+   * If not provided, uses a default key
+   */
+  identifier?: string
+}
+
+export interface RateLimitResult {
+  /**
+   * Whether the request is allowed
+   */
+  allowed: boolean
+  
+  /**
+   * Number of requests remaining in the current window
+   */
+  remaining: number
+  
+  /**
+   * Timestamp when the rate limit will reset
+   */
+  resetAt: number
+  
+  /**
+   * Time in milliseconds until the rate limit resets
+   */
+  retryAfter?: number
+}
+
+/**
+ * Check if a request is allowed based on rate limiting rules
+ * 
+ * @param key - Unique identifier for the rate limit (e.g., "login:user@example.com" or "login:192.168.1.1")
+ * @param config - Rate limit configuration
+ * @returns Rate limit result indicating if request is allowed
+ */
+export function checkRateLimit(
+  key: string,
+  config: RateLimitConfig
+): RateLimitResult {
+  const now = Date.now()
+  const entry = rateLimitStore.get(key)
+  
+  // If no entry exists or the window has expired, create a new entry
+  if (!entry || entry.resetAt < now) {
+    const resetAt = now + config.windowMs
+    rateLimitStore.set(key, {
+      count: 1,
+      resetAt,
+    })
+    
+    return {
+      allowed: true,
+      remaining: config.maxRequests - 1,
+      resetAt,
+    }
+  }
+  
+  // Check if limit has been exceeded
+  if (entry.count >= config.maxRequests) {
+    return {
+      allowed: false,
+      remaining: 0,
+      resetAt: entry.resetAt,
+      retryAfter: entry.resetAt - now,
+    }
+  }
+  
+  // Increment count and allow request
+  entry.count++
+  rateLimitStore.set(key, entry)
+  
+  return {
+    allowed: true,
+    remaining: config.maxRequests - entry.count,
+    resetAt: entry.resetAt,
+  }
+}
+
+/**
+ * Reset rate limit for a specific key
+ * Useful for clearing rate limits after successful authentication
+ * 
+ * @param key - Unique identifier for the rate limit
+ */
+export function resetRateLimit(key: string): void {
+  rateLimitStore.delete(key)
+}
+
+/**
+ * Rate limit configuration for login attempts
+ * Allows 5 login attempts per 15 minutes per email address
+ */
+export const LOGIN_RATE_LIMIT: Omit<RateLimitConfig, 'identifier'> = {
+  maxRequests: 5,
+  windowMs: 15 * 60 * 1000, // 15 minutes
+}
+
+/**
+ * Rate limit configuration for registration attempts
+ * Allows 3 registration attempts per hour per IP address
+ */
+export const REGISTER_RATE_LIMIT: Omit<RateLimitConfig, 'identifier'> = {
+  maxRequests: 3,
+  windowMs: 60 * 60 * 1000, // 1 hour
+}
+
+/**
+ * Rate limit configuration for password reset attempts
+ * Allows 3 password reset attempts per hour per email
+ */
+export const PASSWORD_RESET_RATE_LIMIT: Omit<RateLimitConfig, 'identifier'> = {
+  maxRequests: 3,
+  windowMs: 60 * 60 * 1000, // 1 hour
+}
+
+/**
+ * Rate limit configuration for invitation creation
+ * Allows 10 invitations per hour per user
+ */
+export const INVITATION_RATE_LIMIT: Omit<RateLimitConfig, 'identifier'> = {
+  maxRequests: 10,
+  windowMs: 60 * 60 * 1000, // 1 hour
+}
+
+/**
+ * Get client identifier from request headers
+ * Uses X-Forwarded-For header if available, otherwise falls back to a default
+ * 
+ * @param headers - Request headers
+ * @returns Client identifier (IP address or default)
+ */
+export function getClientIdentifier(headers: Headers): string {
+  // Try to get IP from X-Forwarded-For header (for proxied requests)
+  const forwardedFor = headers.get('x-forwarded-for')
+  if (forwardedFor) {
+    // X-Forwarded-For can contain multiple IPs, use the first one
+    return forwardedFor.split(',')[0].trim()
+  }
+  
+  // Try to get IP from X-Real-IP header
+  const realIp = headers.get('x-real-ip')
+  if (realIp) {
+    return realIp
+  }
+  
+  // Fallback to a default identifier
+  // In production, you might want to handle this differently
+  return 'unknown'
+}